1. Field of the Invention
This invention relates to molded case circuit breakers and, more particularly to an arc runner assembly disposed adjacent the separable main contacts which allows the circuit breaker to interrupt a relatively higher overcurrent.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,642,430 and 4,642,431. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and a relatively high level short circuit condition. An overload condition is about two hundred to three hundred percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000% or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable main contacts which may be operated either manually by way of a handle disposed on the outside of the case, or automatically in response to an overcurrent condition. In the automatic mode of operation, the main contacts may be opened by an operating mechanism or by magnetic repulsion forces. The magnetic repulsion forces cause the main contacts to separate under relatively high level short circuit conditions. In many known circuit breakers, such as disclosed in U.S. Pat. Nos. 4,642,430 and 4,642,431, the separable main contacts are carried by pivotally mounted upper and lower contact arms. More specifically, the upper main contact is carried by a pivotally mounted upper contact arm connected to a load side conductor. The lower main contact is carried by a U-shaped contact arm pivotally mounted with respect to the line side conductor of the circuit breaker. The U-shaped configuration of the lower contact arm causes a reversing of the direction of current flow in the lower contact arm with respect to the upper contact arm, thereby creating magnetic repulsion forces during a relatively high level short circuit condition. These magnetic repulsion forces cause the lower contact arm to rotate downwardly and also cause the upper contact arm to be blown open.
The separation of the upper and lower main contacts generates an electrical arc. The electrical arc is quenched by an arc chute assembly, disposed adjacent the main contacts. In such an arrangement, however, some of the energy in the electrical arc is dispersed by the pivotal movement of the lower contact arm assembly. Specifically, the downward movement of the lower contact arm to rotate downwardly, provides for adequate electrical separation between the lower main contact and the upper main contact. However, such a pivotal lower contact arm is a relatively complicated structure and is also relatively expensive, as compared with molded case circuit breakers used to interrupt relatively lower level overcurrents. For example, in the latter mentioned circuit breakers, the lower contact arm is formed as a U-shape member and is rigidly mounted to the circuit breaker base. Such an arrangement provides for a relatively less complicated and less expensive circuit breaker. However, a rigidly mounted lower contact arm assembly has heretofore not been known to be used at overcurrent levels which currently utilize a pivotally mounted lower contact arm assembly.